Plasticity and optical properties of GaN under highly localized nanoindentation stress fields

Nanoscale plasticity has been studied on (0001) GaN thin films, using tips with very small radius of curvature. Cross-section transmission electron microscopy images of the nanoindentations indicate that the primary slip systems are the pyramidal { 1 1 ¯ 01 } ⟨ 11 2 ¯ 3 ⟩ and { 11 2 ¯ 2 } ⟨ 11 2 ¯ 3 ⟩ , followed by the basal { 0002 } ⟨ 11 2 ¯ 0 ⟩ . Incipient plasticity was observed to be initiated by metastable atomic-scale slip events that occur as the crystal conforms to the shape of the tip. Large volumetric material displacements along the { 1 1 ¯ 01 } ⟨ 11 2 ¯ 3 ⟩ and { 11 2 ¯ 2 } ⟨ 11 2 ¯ 3 ⟩ slip systems were observed at an average shear stress of 11 GPa. Hexagonal shaped nanoindentation impressions following the symmetry of GaN were observed, with material pile-up in the ⟨ 11 2 ¯ 0 ⟩ directions. Spatially resolved cathodoluminescence images were used to correlate the microstructure with the optical properties. A large number of non-radiative defects were observed directly below the indentation. Regions under tensile stress extending from the nanoindentation along ⟨ 11 2 ¯ 0 ⟩ directions were associated with the { 0002 } ⟨ 11 2 ¯ 0 ⟩ slip.